In most of applications of power switch, there is a need of electric insulation between the input and the output to protect a user from a danger of an accident due to high voltage or leakage current. A high-frequency transformer is used for the insulation and this converter is called an ‘isolated DC-DC converter’. The transformer adjusts the magnitude of the output voltage, using the winding ratio of the primary and secondary cables, other than insulating. A flyback converter, a forward converter, a push-pull converter, a half-bridge converter, and a full-bridge converter are typical isolated converters. The flyback converter and the forward converter are generally used for small power circuits under hundreds of watts.
The low-priced converters of the related art use indirect measurement that senses output voltage or current at the primary side of a transformer and estimate secondary (output) information, using an auxiliary winding wound around the same core wound with a primary winding and a secondary winding. However, leakage inductance is caused by poor combination of the primary winding, the secondary winding, and the auxiliary winding, so there is a problem in that it is difficult to accurately sense the output voltage or current.
On the other hand, the transformers for minimizing leakage inductance in the related art have large thicknesses because they have several pieces of inner conductors connected in parallel, such that not a solenoid winding type, but a loop winding type is used and it is wound on a toroidal core that is a ring-shaped magnetic core. Transformers having this structure have been generally used only for large capacity due to the high manufacturing cost and the coupling coefficient is not high because the inner conductors are not twisted.
The background art of the present invention has been described in Korean Patent Application Publication No. 2002-0074203 (2002 09 28).